A wavefront passing the eye will be influenced by the optical parts of the eye such that for example chromatic aberration is provided to the wavefront. The reason is that the refractive indices of the materials in the optical parts of the eye differ for different wavelengths. Thus light having different wavelengths will be refracted a different amount and they will fall on the retina at different places, i.e. different colors can not be focused to the same point. This is called chromatic aberration.
Recently there has been much interest in the correction of the monochromatic aberrations of the eye. It has been revealed that when all monochromatic aberrations are corrected in the human visual system, it serves to unmask the chromatic aberration of the eye, see Yoon G. and Williams D. R.: “Visual performance after correcting the monochromatic and chromatic aberrations of the eye”. J. Opt. Soc. Am. A, 19, 266-275 (2002). Therefore, in order to optimize the optical quality of the eye, a combination of monochromatic and chromatic aberrations needs to be corrected. A diffractive pattern could be configured to provide a passing wavefront with chromatic aberration of the opposite sign as chromatic aberration from the eye. Thus a diffractive pattern can be used to correct for chromatic aberration introduced to a wavefront from the optical parts of the eye.
Some background theory of chromatic aberration can be found in, for example Chapter 17 in “Optics of the Human Eye” written by David A. Atchison and George Smith. A theoretical background of the diffractive pattern could be found in the article “Practical design of a bifocal hologram contact lens or intraocular lens”, Allen L. Cohen, Applied Optics 31(19)(1992).
Ophthalmic lenses, which on at least one surface comprises a diffractive pattern for correcting for chromatic aberration are known from for example U.S. Pat. Nos. 5,895,422, 5,117,306 and 5,895,422. These lenses do, however not, compensate for other aberrations provided by the eye surfaces. WO 01/89424 teaches methods how to design aspheric lenses that compensate for spherical aberration. However, with some applications these lenses will provide the eye with an increase in chromatic aberration. It is therefore a need of an ophthalmic lens for correcting refractive errors that also can correct for monochromatic and chromatic aberrations.
The chromatic aberration of the eye could be measured by using vernier methods such as those similar to the methods outlined in Thibos et. al., “Theory and measurement of ocular chromatic aberration”, Vision Res., 30, 33-49 (1990) and Marcos et. al, Vision Research, 39, 4309-4323, (1999). Alternative ways for measuring chromatic aberration are described in a textbook, “Optics of the Human Eye” by David A. Atchison and George Smith, published by Butterworth-Heinemann, ISBN 0-7506-3775-7.
The longitudinal chromatic aberration of the eye is very well understood and has been shown to have very similar values from subject to subject (Thibos et. al., “The chromatic eye: a new reduced eye model of ocular chromatic aberration in humans”, Applied Optic, 31, 3594-3600, (1992)). It has also been shown to be stable with age (Mordi et. al., “Influence of age on chromatic aberration of the human eye”, Amer. J. Optom. Physiol. Opt., 62, 864-869 (1985)). Hereby an ophthalmic lens to correct for the average chromatic aberration of the eye could be designed.
WO 02/084381 describes a method of how design a hybrid refractive/diffractive ophthalmic lens, wherein chromatic aberration will be reduced by means of the diffractive element, while spherical aberration will be reduced through the use of an aspheric element. Also U.S. Pat. No. 6,338,559 suggests lenses that reduce monochromatic and chromatic aberrations principally by using an apodization filter. Although these efforts are significant, there are still needs to further improve chromatic aberration reduction and the visual quality while adapting the lenses further optimized to meet the needs of the human eye. The present invention aims at meeting such requirements with a hybrid type lens.